2,572 research outputs found
Gas and Dust Dynamics in Starlight-heated Protoplanetary Disks
Theoretical models of the ionization state in protoplanetary disks suggest
the existence of large areas with low ionization and weak coupling between the
gas and magnetic fields. In this regime hydrodynamical instabilities may become
important. In this work we investigate the gas and dust structure and dynamics
for a typical T Tauri system under the influence of the vertical shear
instability (VSI). We use global 3D radiation hydrodynamics simulations
covering all of azimuth with embedded particles of 0.1 and 1mm
size, evolved for 400 orbits. Stellar irradiation heating is included with
opacities for 0.1- to 10-m-sized dust. Saturated VSI turbulence produces a
stress-to-pressure ratio of . The value of is
lowest within 30~au of the star, where thermal relaxation is slower relative to
the orbital period and approaches the rate below which VSI is cut off. The rise
in from 20 to 30~au causes a dip in the surface density near 35~au,
leading to Rossby wave instability and the generation of a stationary,
long-lived vortex spanning about 4~au in radius and 40~au in azimuth. Our
results confirm previous findings that mm size grains are strongly vertically
mixed by the VSI. The scale height aspect ratio for 1mm grains is determined to
be 0.037, much higher than the value obtained from millimeter-wave
observations of the HL~Tau system. The measured aspect ratio is better fit by
non-ideal MHD models. In our VSI turbulence model, the mm grains drift radially
inwards and many are trapped and concentrated inside the vortex. The turbulence
induces a velocity dispersion of ~m/s for the mm grains, indicating
that grain-grain collisions could lead to fragmentation.Comment: ApJ accepte
Global Hydromagnetic Simulations of Protoplanetary Disks with Stellar Irradiation and Simplified Thermochemistry
Outflows driven by large-scale magnetic fields likely play an important role
in the evolution and dispersal of protoplanetary disks, and in setting the
conditions for planet formation. We extend our 2-D axisymmetric non-ideal MHD
model of these outflows by incorporating radiative transfer and simplified
thermochemistry, with the twin aims of exploring how heating influences wind
launching, and illustrating how such models can be tested through observations
of diagnostic spectral lines. Our model disks launch magnetocentrifugal
outflows primarily through magnetic tension forces, so the mass-loss rate
increases only moderately when thermochemical effects are switched on. For
typical field strengths, thermochemical and irradiation heating are more
important than magnetic dissipation. We furthermore find that the entrained
vertical magnetic flux diffuses out of the disk on secular timescales as a
result of non-ideal MHD. Through post-processing line radiative transfer, we
demonstrate that spectral line intensities and moment-1 maps of atomic oxygen,
the HCN molecule, and other species show potentially observable differences
between a model with a magnetically driven outflow and one with a weaker,
photoevaporative outflow. In particular, the line shapes and velocity
asymmetries in the moment-1 maps could enable the identification of outflows
emanating from the disk surface.Comment: 35 pages, 20 figures, accepted for publication in Ap
Characterization of extrasolar terrestrial planets from diurnal photometric variability
The detection of massive planets orbiting nearby stars has become almost
routine, but current techniques are as yet unable to detect terrestrial planets
with masses comparable to the Earth's. Future space-based observatories to
detect Earth-like planets are being planned. Terrestrial planets orbiting in
the habitable zones of stars-where planetary surface conditions are compatible
with the presence of liquid water-are of enormous interest because they might
have global environments similar to Earth's and even harbor life. The light
scattered by such a planet will vary in intensity and colour as the planet
rotates; the resulting light curve will contain information about the planet's
properties. Here we report a model that predicts features that should be
discernible in light curves obtained by low-precision photometry. For
extrasolar planets similar to Earth we expect daily flux variations up to
hundreds of percent, depending sensitively on ice and cloud cover. Qualitative
changes in surface or climate generate significant changes in the predicted
light curves. This work suggests that the meteorological variability and the
rotation period of an Earth-like planet could be derived from photometric
observations. Other properties such as the composition of the surface (e.g.,
ocean versus land fraction), climate indicators (for example ice and cloud
cover), and perhaps even signatures of Earth-like plant life could be
constrained or possibly, with further study, even uniquely determined.Comment: Published in Nature. 9 pages including 3 figure
Black Stork Down: Military Discourses in Bird Conservation in Malta
Tensions between Maltese hunters and bird conservation NGOs have intensified over the past decade. Conservation NGOs have become frustrated with the Maltese State for conceding to the hunter lobby and negotiating derogations from the European Union’s Bird Directive. Some NGOs have recently started to organize complex field-operations where volunteers are trained to patrol the landscape, operate drones and other surveillance technologies, detect illegalities, and lead police teams to arrest poachers. We describe the sophisticated military metaphors which conservation NGOs have developed to describe, guide and legitimize their efforts to the Maltese public and their fee-paying members. We also discuss why such groups might be inclined to adopt these metaphors. Finally, we suggest that anthropological studies of discourse could help understand delicate contexts such as this where conservation NGOs, hunting associations and the State have ended in political deadlock
Denitrification and nitrous oxide emissions from riparian forests soils exposed to prolonged nitrogen runoff
Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO3) from agricultural run-off through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N2O) emissions from riparian forest soils exposed to prolonged nutrient run-off from plant nurseries and compares these to similar forest soils not exposed to nutrient run-off. Nursery run-off also contains high levels of phosphate (PO4). Since there are conflicting reports on the impact of PO4 on the activity of denitrifying microbes, the impact of PO4 on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N2O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 μg NO3-N g-1 soil, respectively. Net N2O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 μg NO3-N g-1 soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 μg NO3-N g-1 in soil slurries. The addition of PO4 (5 μg PO4–P g-1) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N2O emissions compared to emission rates from non-exposed forests
Characterization of a Lamellocyte Transcriptional Enhancer Located within the misshapen Gene of Drosophila melanogaster
Drosophila has emerged as an excellent model system in which to study cellular and genetic aspects of hematopoiesis. Under normal developmental conditions and in wild-type genetic backgrounds, Drosophila possesses two types of blood cells, crystal cells and plasmatocytes. Upon infestation by a parasitic wasp or in certain altered genetic backgrounds, a third hemocyte class called the lamellocyte becomes apparent. Herein we describe the characterization of a novel transcriptional regulatory module, a lamellocyte-active enhancer of the misshapen gene. This transcriptional control sequence appears to be inactive in all cell types of the wild-type larva, including crystal cells and plasmatocytes. However, in lamellocytes induced by wasp infestation or by particular genetic conditions, the enhancer is activated and it directs reporter GFP or DsRed expression exclusively in lamellocytes. The lamellocyte control region was delimited to a 140-bp intronic sequence that contains an essential DNA recognition element for the AP-1 transcription factor. Additionally, mutation of the kayak gene encoding the dFos subunit of AP-1 led to a strong suppression of lamellocyte production in tumorous larvae. As misshapen encodes a protein kinase within the Jun N-terminal kinase signaling pathway that functions to form an active AP-1 complex, the lamellocyte-active enhancer likely serves as a transcriptional target within a genetic auto-regulatory circuit that promotes the production of lamellocytes in immune-challenged or genetically- compromised animals
Fit for Birth- the effect of maternal weight changes in obese pregnant women on maternal and neonatal outcomes.
The ‘Fit for Birth’ study aimed to explore patterns of gestational weight gain and their relationship with pregnancy outcomes.
The study had three aims:
1. To explore the feasibility of conducting a large cohort study in this setting.
2. To describe patterns of weight gain through pregnancy in obese women.
3. To explore associations of weight change during pregnancy with outcomes.
Study population
Pregnant women with a BMI ≥ 30 kg m−2 at first antenatal clinic visit.
Methods
This was a single centre pilot observational study based at the Liverpool Women's Hospital, a large UK maternity hospital.Women were recruited into the study at their antenatal booking visit and had weights measured throughout pregnancy. Patterns of weight gain were described and related to maternal and neonatal outcomes.
Main outcome measure
The primary outcome was a composite measure consisting of any of 12 adverse maternal and foetal outcomes. This was compared by categorized pregnancy weight gain (9 kg).
Results
Eight hundred and twenty four women consented to participation between June 2009 and June 2010. Weight data were collected on 756 women. Only 385 women had weights measured in all three study assessment periods (6–20 weeks, 20 + 1 to 32 weeks and >32 weeks gestation) while 427 women had weights measured in period 3. Individual patterns of weight gain varied widely and missing data were common and non‐random. There was a significant association between increased weight gain during pregnancy and poor maternal and foetal outcome.
Conclusions
Weight gain in obese women during pregnancy can be highly variable. Our study supports an association between increased weight gain in pregnancy and adverse perinatal outcomes
In the Shadow of the Transiting Disk: Imaging epsilon Aurigae in Eclipse
Eclipses of the single-line spectroscopic binary star, epsilon Aurigae,
provide an opportunity to study the poorly-defined companion. We used the MIRC
beam combiner on the CHARA array to create interferometric images during
eclipse ingress. Our results demonstrate that the eclipsing body is a dark disk
that is opaque and tilted, and therefore exclude alternative models for the
system. These data constrain the geometry and masses of the components,
providing evidence that the F-star is not a massive supergiant star.Comment: As submitted to Nature. Published in Nature April 8, 2010
Analysis of CC chemokine and chemokine receptor expression in solid ovarian tumours
To understand the chemokine network in a tissue, both chemokine and chemokine receptor expression should be studied. Human epithelial ovarian tumours express a range of chemokines but little is known about the expression and localisation of chemokine receptors. With the aim of understanding chemokine action in this cancer, we investigated receptors for CC–chemokines and their ligands in 25 biopsies of human ovarian cancer. CC–chemokine receptor mRNA was generally absent from solid tumours, the exception being CCR1 which was detected in samples from 75% of patients. CCR1 mRNA localised to macrophages and lymphocytes and there was a correlation between numbers of CD8+ and CCR1 expressing cells (P = 0.031). mRNA for 6 CC-chemokines was expressed in a majority of tumour samples. In a monocytic cell line in vitro, we found that CCR1 mRNA expression was increased 5-fold by hypoxia. We suggest that the CC-chemokine network in ovarian cancer is controlled at the level of CC-chemokine receptors and this may account for the phenotypes of infiltrating cells found in these tumours. The leukocyte infiltrate may contribute to tumour growth and spread by providing growth survival factors and matrix metalloproteases. Thus, CCR1 may be a novel therapeutic target in ovarian cancer. http://www.bjcancer.com © 2001 Cancer Research Campaignhttp://www.bjcancer.co
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